Considerable interest exists in the development of flame-retardant polymers for a wide range of applications. Most conventional organic polymers have useful structural and mechanical properties, but are limited by their low thermo-oxidative stability. A typical example is found in polyurethanes, the many varieties of which are widely used in the aerospace and construction industries. However, polyurethanes are highly combustible. As a result, their thermal stability has been studied extensively. Three general approaches exist to reducing the flammability of polyurethanes; (1) the addition of small molecule flame retardants; (2) modification of the polyurethane structure to influence the thermal decomposition pathways; and (3) blending with other polymer systems to enhance the thermo-oxidative stability of the resulting material. Each approach has its advantages and disadvantages. The first approach, the addition of small molecule flame retardants, is relatively easy and inexpensive but suffers from the problem of migration or leaching of the small molecules out of the system. This limits their effectiveness over long periods of time. In addition, the small molecule additives may influence the decomposition reactions in a way that increases the production of smoke and toxic vapors, thus hampering emergency countermeasures and increasing the likelihood of death by inhalation. The second approach, the modification of the polymer structure, is often higher in cost and can compromise the mechanical properties. The third approach, which involves blending of the polyurethane with other polymers, could utilize a polymer that inhibits combustion by modifying the decomposition mechanism of the polyurethane, by the release of noncombustable gases, and/or by undergoing reactions during heating to create a high char yield to quench further combustion. However, the choice of suitable polymeric flame-retardants is restricted to species that allow retention of advantageous mechanical properties of the polyurethane.
In considering this third approach, flame retardant polymer blends, the phase behavior of polymer-polymer mixtures is crucial, because it strongly influences the chemical, physical and mechanical properties. Conditions governing the mixing of polymers are stringent and, it is a general rule that the probability is exceedingly low that a single phase (miscible) mixture of two randomly chosen high molar mass polymers can be obtained. Accordingly, the mixing of two polymers usually results in a grossly phase separated material. Macromolecular compatibilizers (usually block or graft copolymers) have been used to decrease the domain size of the dispersed phase and produce a more homogeneous dispersion, but ideally intimate mixing, exemplified by a single phase (miscible) mixture, is preferred. Miscible polyurethane blends are sparse, but can be designed by incorporating functional groups capable of specific intermolecular interactions. In situ chemical reactions between the two polymers in the mixture is another method to ensure intimate mixing, and, given the appropriate chemistry, numerous graft and interpenetrating networks can be designed. The solution that we present here is the use of a class of readily modified macromolecules known as poly(organophosphazenes).
Poly(organophosphazenes) form a large class of macromolecules with the general formula (NPR.sub.2).sub.n. They contain alternating nitrogen and phosphorus atoms in the backbone and contain a wide variety of organic, organometallic, and inorganic side groups. Specific poly(organophosphazenes) have high resistance to solvents, low temperature flexibility, and good thermal stability. The primary synthesis route is through the ring-opening polymerization of molten hexachlorocyclotriphosphazene at 250.degree. C. to form poly(dichlorophosphazene). The phosphorus-chlorine bonds in this polymer are highly responsive to macromolecular substitution by a wide range of nucleophiles to give a broad spectrum of poly(organophosphazenes). These have a wide range of properties that depend on the nature of the side groups. The polyphosphazenes useful in the instant invention are prepared according to standard techniques known to those skilled in the art of polyphosphazene preparation. Methods of preparation are further discussed in U.S. Pat. No. 4,880,622, to Allcock et al., U.S. Pat. No. 5,053,451 to Allcock et al., and U.S. No. 5,457,160 to Allcock et al (all incorporated herein by reference).
The thermo-oxidative stability at elevated temperatures is of primary interest when tailoring poly(organophosphazenes) for flame-retardant applications. The thermo-oxidative stability of one poly(organophosphazene) in particular, poly[bis(carboxylatophenoxy) phosphazene] has been examined [Reed, C. S. et al. Chem. Mater. 1996, 8, 440]. It has features which make it a good candidate for blending with polyurethanes. It was found that it undergoes cross-linking at temperatures above 200.degree. C. with a high char yield which could quench further combustion.
Polymer blends or alloys are physical mixtures of two or more polymers. The phase behavior of polymer blends is determined essentially by the balance between unfavorable "physical" forces expressed in terms of solubility parameter differences and favorable "chemical" forces which are derived from intermolecular specific interactions. The strength of the interactions depends on the functional groups in the macromolecules. The roles played by the different functional groups in these interactions has been reported in detail for organic polymers [Coleman, M.; Graf, J. F.; Painter, P. C. Specific Interactions and the Miscibility of Polymer Blends; Technomics Publishing, Lancaster, Pa. 1991; Olabisi, P.; Robeson, L. M.; Shaw, M. T. Polymer-Polymer Miscibility; Academic Press: New York, 1979, p. 207 and references therein].
Polymer blends can exist as miscible one-phase systems, as semimiscible systems that have domains which exist together with phases rich in one of the constituent polymers, or as immiscible multi-phase materials systems. Many examples exist of miscible and immiscible polymer blends. An example of a miscible blend is that between polycaprolactone and poly(vinyl chloride), in which the driving force for miscibility is apparently the close matching of the solubility parameter and the presence of intermolecular hydrogen bonding between the Cl-C-H unit of the poly(vinyl chloride) and the proton-accepting character of the carbonyl group of the polycaprolactone [Coleman, M.; Graf, J. F.; Painter, P. C. Specific Interactions and the Miscibility of Polymer Blends; Technomics Publishing, Lancaster, Pa. 1991; Olabisi, O. Macromolecules 1975, 8, 316].
In the thesis of Paul E. Austin (Polyphosphazenes, New Biomaterials, Penn State University, 1984), the following polymer blends were reported: (i) [NP(OCH.sub.2 CF.sub.3).sub.2 ].sub.n (referred to as poly[bis(trifluoroethoxy)phosphazene]) with poly(methylmethacrylate), poly(vinylpyrrolidone), and phenoxy resin; and (ii) [NP(OCH3)2]n (referred to as poly[bis(methylamino)phosphazene]) with poly(vinylpyrrolidone), poly(acrylic acid), poly(vinyl alcohol), and methyl cellulose. This disclosure does not mention or suggest the possibility of blends containing polyurethane.